1. Field of the Invention
The present invention relates to a laser scanning unit for an image forming device, and more particularly, to a tandem type laser scanning unit that scans a plurality of light sources over one deflecting device and projects images on a plurality of photoconductive drums.
2. Description of the Related Art
In general, a laser scanning unit (LSU) is used in image forming devices such as copiers, printers, or facsimile machines, all of which print images on printing paper. The LSU scans a light beam emitted from a light source, for example, a laser diode, to a photoconductive medium of the image forming device and makes an electrostatic latent image.
Laser scanning units can be described as comprised of two different optical systems, both of which are dependent upon the polygon mirror. The first system is an incident optical system, and the second is an image optical system. The incident optical system comprises the area of the LSU from the light source to the polygon mirror. The image optical system comprise the area of the LSU from the polygon mirror to the photoconductive drum.
The incident optical system includes a light source which emits a light beam, a collimating lens that directly transmits the light beam emitted from the light source, and a polygon mirror that reflects the light beam. The light source can include a plurality of laser semiconductors, the plurality of which emits a plurality of light beams.
A tandem type LSU projects the plurality of light beams emitted from a plurality of light sources onto a plurality of photoconductive drums. The light beams emitted from the light sources are projected onto a polygon mirror with different incidence angles with respect to a main scanning direction and are respectively reflected onto the photoconductive drums. Because of the different incident angles, errors in linearity can occur.
The LSU that solves this problem is disclosed in Japanese Patent Publication No. 2000-147399. the entire contents of which are incorporated herein by reference. This LSU changes proceeding paths of light beams emitted from multiple light sources and projects the beams onto the polygon mirror with the same incidence angles. It is difficult to use the plurality of light sources, however, to project the light beams emitted from each light source onto the polygon mirror with the same incidence angle without a loss of luminous intensity. In addition, changes in focal distances that result from temperature shifts generally affects the optical performance of the LSU.